Orthogonal Frequency Divisional Multiple Access (QFDMA) is a multi-carrier modulation technique that effectively partitions overall system bandwidth into multiple orthogonal sub-bands. These sub-bands are also referred to as tones, sub-carriers, bins and frequency channels. In OFDM communication systems, each sub-band or frequency channel is associated with a respective sub-carrier that may be modulated with data.
In an OFDM communication system, an RF modulated signal from a transmitter may reach a receiver via a number of propagation paths. Characteristics of the propagation paths typically vary over time due to a number of factors such as fading and multi-paths. To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used. If the propagation paths between the transmit and receive antennas are linearly independent, that is, a transmission on one path is not formed as a linear combination of the transmissions on the other paths, in the likelihood of correctly receiving a data transmission increases as the number of antennas increase.
A MIMO communication system employs multiple transmit antennas and multiple receive antennas for data transmission. A MIMO channel formed by the transmit and receive antennas may be decomposed into multiple independent layers each of which corresponds to a different transmit/receive antenna pair. Each of the independent layers may also be referred to as a spatial sub-channel or layer. In order to increase throughput, a pre-coding matrix is applied to the layers prior to transmission.
In OFDM communication systems, feedback about the quality of the downlink channel between a transmitter and a receiver is employed in order to optimize the rate of transmission of data over the downlink channel. The receiver determines the downlink channel quality information from received pilot signals, and then communicates this downlink channel quality information to the transceiver for use in altering the pre-coding matrices, coding rate and modulation scheme of transmission used in subsequent data transmission. Whilst this downlink channel quality information is used to improve the transmission capacity of MIMO communication systems, future communication systems will require even further improvements to be made in transmission capacity due to expected increases in transmitted data volumes and increasing user expectations.